Add mysql gem version string for activerecord not to install gem newer than 2.8

- this fix will be reverted when Rails 4 released
Merge pull request #993 from ujifgc/master

Fixing #818
Merge pull request #989 from udzura/fix/patch-1

Add mysql gem version string for activerecord
one small issue
Merge pull request #995 from ishe-ua/master

one small issue
Merge branch 'master' of git://github.com/padrino/padrino-framework into groups